Resistor



United ates Patent 3,142,811 RESISTOR James A. OConnell, Syosset, N.Y., assignor to General Telephone and Electronics Laboratories, Inc., a corporation of Delaware Filed Nov. 29, 1961, Ser. No. 155,622 8 Claims. (Cl. 338-20) My invention relates to electrical resistors and, in particular, to resistors having non-linear voltage-current characteristics.

Non-linear resistors are manufactured in a variety of forms and are utilized in many types of electrical and electronic circuits. However, these known non-linear resistors are not generally satisfactory for low voltage D.C. applications or for operation at the current and voltage levels encountered in many transistor circuits. Accordingly, I have invented an improved low impedance nonlinear resistor of the type wherein the resistance decreases as the voltage applied across the resistor increases.

It is an'object of my invention to provide a non-linear resistor which is suitable for operation at low direct voltages.

It is another object of the invention to provide a nonlinear. resistor which is highly reproducible, easy to fabricate and inexpensive to make.

Still another object is to provide a non-linear resistor which has a relatively high current conductivity and a relatively high slope.

A further object is to provide a non-linear resistor which is compatible with low voltage D.C. transistorized circuits.

' In accordance with the invention, first and second spaced apart electrodes are secured to a layer of tin protoxide (SnO) subtended between the electrodes. When a direct voltage is applied across the electrodes and the magnitude of theivoltage increased, the magnitude of the current through the layer increases in a non-linear manner. In particular, a small increase in voltage results in a relatively large increase in current and consequently a sharp decrease in resistance. Stated another way, the current through the layer equals KV where V is the applied voltage, n is a number greater than 1, and K is a constant of proportionality. y V

* In one method of preparing my non-linear resistor, tin protoxide powder is admixed with a solvent having a relatively high evaporation rate, such as acetone, and the mixture poured into a vessel containing an electroded substrate. After the tin protoxide has settled on tothe substrate, most of the acetone is withdrawn and a small amount of epoxy resin containing a curing agent dissolved in the remaining fluid. The system is then evaporated to dryness, the sample cured in an oven, and a second electrode evaporated on to the top surface of the tin protoxide layer.

It has been found that the conductivity of the nonlinear resistor can be increased by heating the tin protoxide powder in air prior to settling on to the substrate. Further, the intrinsic slope of the resistor can be increased by coating the tin protoxide with a small amount of silicon dioxide (SiO deposited from suspension.

The above objects of and the brief introduction to the present invention will be more fully understood and further objects and advantages will become apparent from a study of the following description in connection with the drawings, wherein:

FIG. 1 is a cross-sectional view of one type of nonlinear resistor in accordance with the invention,

FIG. 2 is a cross-sectional view of another type of nonlinear resistor in accordance with the invention, and

FIG. 3 is a log-log graph of the current voltage charac- 3,142,81i Patented July 28, 1964 "ice teristics of nonlinear resistors made in accordance with the invention.

Referring to FIG. 1, there is shown a glass plate 10, one

surface of which is covered With a transparent electrically conductive tin oxide film 11. A layer 12 of tin protoxide (SnO) is applied over the film 11 and a second conductive tin oxide film 13 is applied over layer 12. In lieu of glass plate 10 and tin oxide film 11, any inert electroded substrate material may be used, such as a copper clad plastic laminate. Also other conducting materials, such as aluminum, copper, silver, nickel or tin may be used for the top electrode 13.

i In one method of preparing my non-linear resistor, one gram of tin protoxide powder (Type T150 obtained from the Fisher Scientific Co., New York, N.Y.) is admixed with cubic centimeter of acetone. The mixture is then agitated and poured over a one square inch electroded glass substrate 10, 11 placed in the bottom of a milliliter settling beaker. After the tin protoxide powder has uniformly settled over the substrate, 60 cubic centimeter of the acetone are withdrawn and about 2.3 cubic centimeter of an epoxy resin solution containing 100 cubic centimer of acetone, 1.8 grams of epoxy resin, and 0.234 gram of a catalyst are added to the beaker. (An epoxy resin which has been found highly satisfactory for this purpose is known by the tradename Epon 828 and is sold by the Shell Chemical Company, New York, N.Y. The catalyst used with this resin is a polyamine known as Epon Curing Agent D.) The system is next evaporated to dryness and the area of the non-linear resistor layer reduced to the desired size by scraping away the superfluous resistive material. The sample is then cured in an oven heated to 100 C. for 90 minutes after which the top electrode 13 is evaporated onto the non-linear resistive layer 12. The epoxy resin serves to bond the particles of the tin protoxide powder together to provide a non-linear resistor having easily reproducible characteristics.

Curve 20 of FIG. 3 illustrates the current-voltage characteristic of a non-linear resistor made by the above method in which the area of the non-linear resistive layer is 0.1875 square inch and the thickness of the layer is 0.006 inch. The current in this resistor changes from 0.02 milliampere with 7 volts D.C. applied across it to 3.5 milliamperes at 17 volts. The slope n of the characteristic curve is approximately 7.1 and the current density J in amperes per square inch may be expressed as where E is in volts per mil. Curve 21 shows the voltagecurrent characteristic of another sample of the same thickness but having an area of 0.0156 square inch. The current density for this sample is given by ]=4.75 10" E amperes per square inch where the slope n equals 5.

The percentage of epoxy resin and curing agent used in the system affects the slope and conductivity of the non-linear resistor. Excellent results have been obtained by varying the percentage of epoxy resin and catalyst between 2% and 5% by weight of tin protoxide powder. In general, with the smaller amounts of resin, the slope of the resistor is relatively low and the conductivity relatively high while, at the high end of the range, the converse is true. Curve 22 shows the voltage-current characteristic for a non-linear resistor having an area of 0.1875 square inch, a thickness of about 0.006 inch and 2% epoxy resin by weight of tin protoxide. Curve 23 illustrates the voltage-current characteristics of a nonlinear resistor of the same dimensions in which the percentage of epoxy resin and catalyst are 5% by weight of tin protoxide.

It has been found that the non-linear resistor can be made more conductive by heating the tin protoxide powder in air at 200-400 C. for about 40 minutes in a nickelcrucible. Curve 24 is obtained by heating tin protoxide powder to 350 C. for 40 minutes and then using the powder to prepare a non-linear resistor in the manner previously described. It shall be noted that while the conductivity is higher, the slope n of the character istic curve is decreased to about 4.0. The area of. the non-linear resistive layer is 0.1875 square inch, the thickness 0.006 inch, and the percentage ofepoxy resin and catalyst is 5% of the weight of tin protoxide powder.

The slope of the resistor may be increased by coating the polycrystalline tin protoxide with a small amount of insulating silicon dioxide deposited from suspension. Specifically, the process consists of heating tin protoxide powder for about 40 minutes at a temperature of 300- 400 C. and. then washing about 4 grams ,of the powder in 20 cubic centimeters of acetone containing 0.5 cubic centimeter of a silicon dioxide dispersion such as nalcoag 1036A manufactured by the National Aluminate Company. The powder is next dried, heated for 1 hour at 120 C. and settled in the manner previously indicated. Curve 25 shows the voltage-current characteristicof anon-linear resistor havingv the same dimensions and percentage of epoxy resin as that used to obtain curve 24. However, in curve 25 it is seen thatthe slope n is increased to about 9.3 while the conductivity is decreased.

FIG: 2 shows in cross-section another embodiment of my invention. It comprises a glass substrate 10, first and second coplanar parallel separated brass electrodes 40 and 42, and a layer 44 of tin protoxide powder applied over and between the electrodes. (Alternatively, the layer can be applied between the electrodes and need not cover them.) The two electrodes 40 and 42 can be interdigital orcan take any complex shape as long as they remain separated. The characteristics of nonlinear resistors made in accordance with FIG. 2 are essentially the. samev as those fabricated. as shown in FIG. 1.

While the epoxy resin contributes to the production 4 powder was thoroughly dried. The slope n of this curve is about 4.5.

As many changes could be made in the above construction and many different embodiments could be made Without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A non-linear resistor comprising alayer of tin protoxide and epoxy resin, the particles of said tinprotoxide being bonded together bysaid epoxy resin.

2. A non-linear resistor comprising a layer of tin protoxide and epoxy resin, the particles of said tin protoxide being bonded' together by said epoxy'resin, and'first and second. spaced apart electrodes secured to said layer..

3. A non-linear resistor comprising a layer of tin protoxide and epoxy resin, the particles of said tin protoxide being bonded together by between 2% and 5% epoxy resin by weight of tin protoxide, and first and second spaced apart electrodes secured to said layer.

4'. A non-linear resistor as defined in claim 3 wherein the amount of epoxy resin is 2% by weight of tin prot-. oxide.

5. A non-linear resistor as defined in claim 3 wherein the amount of epoxy resin is 5% by Weight of tin protoxide.

6. A non-linear resistor comprising a layer of tin'protoxide powder having a coating of silicon dioxide deposited" thereon, an epoxy resin bonding the particles of said. coated tin protoxide powder, and first and second electrodes secured to opposite surfaces of said layer.

References Cited in the file of this patent- UNITED STATES PATENTS 2,258,646 Grisdale Oct. 14, 1941' 2,777,044 Lytle Jan. 8, 1957 2,862,088 Mairs Nov. 25, 1958 

8. THE METHOD OF MAKING A NON-LINEAR RESISTOR COMPRISING THE STEPS OF HEATING TIN PROTOXIDE IN AIR TO A TEMPERATURE BETWEEN 200*C. AND 400*C. FOR ABOUT 40 MINUTES, ADMIXING SAID TIN PROTOXIDE POWDER WITH ACETONE, SETTLING THE RESULTING SOLUTION OVER AN ELECTRODED SUBSTRATE AND ALLOWING SAID SOLUTION TO EVAPORATE, AND DEPOSITING AN ELECTRODE OVER THE TOP SURFACE OF THE RESULTING NON-LINEAR RESISTIVE LAYER. 